show or hide the ancestor for a comparison view?
The option to hide ancestor is available only in 3-way mode. The ancestor is still taken into account to make the comparison, this option just gives more place to left and right versions.
Use the icon from the toolbar or the View / Show/Hide ancestor for diff/merge menu item.
If you want to completely remove the ancestor, use the icon from the toolbar or follow the idications in: changing quickly comparisons paths